An RF power amplifier provides the final stage of amplification for a communication signal that has been modulated and converted into an RF signal. The RF signal provided to an RF power amplifier for amplification generally exhibits frequencies confined within a predetermined frequency band licensed by a regulatory agency for a particular use. Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the RF power amplifier to perform a linear amplification operation. The degree to which the amplification is not perfectly linear leads to distortion, unwanted intermodulation, and spectral regrowth in the amplified RF signal output from the RF power amplifier.
Spectral regrowth can cause the amplified RF signal output from the RF power amplifier to spill outside the frequency band where the transmitter is licensed to operate. Other forms of distortion, though nevertheless confined within the licensed frequency band, cause deterioration in an error vector magnitude (EVM) parameter of the amplified RF signal output from the RF power amplifier.
While perfect linearity is a goal for any linear RF power amplifier to minimize the pernicious effects of amplifying with an imperfectly linear transfer function, all linear RF power amplifiers invariably full to meet it. Accordingly, a wide variety of schemes have been developed to improve the linearity of imperfectly linear RF power amplifiers used in RF transmitters. Many of these schemes seek to cancel unwanted signal components in the amplified RF signal. Some of these schemes actually introduce out-of-band signals prior to amplification with the goal of cancelling other out-of-band signals generated as a result of imperfectly linear amplification.
Unfortunately, a typical amplified RF signal is an extremely complex composite of wanted and unwanted signal components. The unwanted signal components result from many different distorting effects. As distortion is forced to be a smaller and smaller part of the amplified RF signal, more and more sources of distortion need to be managed. And, the number of combinations and permutations of different signal components that have interacted with one another by being processed through a nonlinear process is intractable. Accordingly, while many diverse schemes have been directed toward the problem of improving RF amplifier linearity, few have actually achieved a highly linear result.
Another trend in RF transmitters, apart from incorporating schemes to improve linearity, is enhancing power-added efficiency (PAE). Power-added efficiency, hereinafter referred to simply as “efficiency”, is the ratio of the RF output power to the sum of the input RF power and the applied bias power. In conventional RF transmitters, improvements in efficiency have been achieved at the expense of linearity. In other words, linearity has actually been degraded in order to improve efficiency
A variety of RF power amplifier efficiency enhancements has been proposed. These proposals suggest the use of schemes to variably bias the RF power amplifier. Traditionally, “biasing” has referred to DC voltages and currents that are applied to power inputs and signal inputs of amplifiers so that they will reproduce an input signal in a desired manner. The biasing establishes the operating point of the amplifier. Using Lateral Diffusion Metal Oxide Semiconductor (LDMOS) field-effect transistor (FET) terminology, the biasing refers to traditionally DC voltages applied to the drain and gate of an LDMOS, FET, RF power amplifier. But when DC biasing is used, poor efficiency usually results. Accordingly, for variably biased RF power amplifiers, these bias voltages are modulated to achieve improved efficiency with the goal of harming linearity as little as possible. That goal has been difficult to meet.
With the envelope-elimination and restoration (EER) technique, also known as the Kahn technique, the amplitude component of a communication signal is separated from the phase component. Then, the phase component is amplified in a highly efficient amplifier configured for a nonlinear class of operation. The amplitude component is restored by varying the bias voltage at the power input (e.g., the drain) of the nonlinear class amplifier commensurate with the amplitude component of the communication signal. But a significant price is typically paid in linearity by this technique. Accordingly, the EER technique is not readily useable in high power and wide bandwidth applications because, rather than realizing efficiency enhancement, efficiency deterioration is the likely result along with reduced linearity. Efficiency deterioration would result from attempting to generate a high power bias voltage that exhibits a bandwidth consistent with the amplitude content of a wide bandwidth signal.
Another variably biased RF power amplifier technique is the envelope-following technique. Envelope following differs from the EER technique in that both the amplitude and phase components of the communication signal are amplified in a linear-class amplifier. But like the EER technique, power input bias voltage is varied in a manner commensurate with the amplitude content of the communication signal. Accordingly, bias voltage need not be greater than it needs to be to accommodate the RF signal being amplified in a linear class of operation on an instant-by-instant basis. Efficiency enhancements result when compared to traditional linear-class amplifier operation using static DC biasing voltages. Typically, timing issues are less critical than in the EER technique, and the linearity deterioration is less severe than in the EER technique as a result. But a linearity penalty still results, and the envelope-following technique is not readily usable in high power and wide bandwidth applications because, rather than realizing efficiency enhancement, efficiency deterioration is the likely result.
Another variably biased RF power amplifier technique is the envelope-tracking technique. Envelope tracking differs from the envelope-following technique in that the envelope of the RF communication signal is not followed completely. This lowers the switching frequency requirements in the power supply that generates the bias voltage applied to the RF power amplifier's power input, resulting in some efficiency gain to offset an efficiency loss suffered by not completely following the envelope. And, timing issues become less critical, at least in narrow bandwidth applications, so that linearity need not suffer greatly. But a linearity penalty still results, and nothing is provided to ensure that the linearity penalty does not result in the violation of a spectral mask.
The above-referenced Related Inventions discuss yet another variably biased RF power amplifier technique which yields improvements even in a wide bandwidth, high power application. But none of these efficiency techniques are as compatible as desired with a transmitter application that calls for the use of a highly linear or highly linearized RF power amplifier. Accordingly, a need exists for an RF transmitter that better accommodates both variable biasing for its RF power amplifier along with linearization of the RF power amplifier.